Inverter is a device converting a DC current into an AC current, and has a plurality of topological structures, among which three-level topological structure has advantages of large output capacity, high output voltage and small current harmonic content, and is widely applied in the fields of AC motor speed regulation, new energy power generation, energy storage, static var compensation and flexible power transmission. Neutral Point Clamped (NPC) three-level inverter is a common inverter with three-level topological structure, including diode-clamped NPC, T-type NPC, Active Neutral Point Clamped (ANPC) and the like. FIG. 1A shows a three-phase diode-clamped NPC three-level topological structure, of which a DC side is a capacitance bridge arm consisting of two (groups) of capacitors connected in series and having a capacity of C, wherein the upper and lower capacitors may be formed of one or more capacitor devices. The three-level topological structure has a phenomenon of an unbalance of the neutral point voltage, i.e., the voltages of the upper and lower capacitors are not the same. The unbalance issue of the neutral point voltage is always difficult to be solved, and especially under the working conditions of pure reactive, unbalanced and non-linear AC load, the unbalance of the neutral point voltage on the DC side easily occurs. If this phenomenon is not suppressed, then an output voltage becomes poorer, and an output current becomes worse, causing a DC overvoltage or an AC overcurrent, and a shutdown with failure. Therefore, it is of great importance for wide application of the three-level topology to design a reasonable control method to suppress an unbalance of the neutral point voltage.